1. Field of the Invention
The present invention relates to an ultrafast optical switching device utilizing nonlinear optical effects of a multi-quantum well structure (hereinafter, referred to as "MQWS"), and more particularly to an ultrafast optical switching device having a double-junction MQWS into which two types of MQWS's are united so as to obtain a fast response of optical nonlinearity thereof.
2. Description of the Prior Art
Nonlinear optical effects in semiconductors offer a wide range of potential applications in the field of optical information processing. In particular, GaAs/Al.sub.1-x Ga.sub.1-x As quantum well structures, where photocreated electrons of the conduction band and holes of the valence band both find their lowest energetic states in the GaAs layer, exhibit large optical nonlinearities in the vicinity of their band edge. This can be attributed to the enhanced exciton binding energy in two dimensions giving rise to strong and well-resolved excitonic resonances in the absorption spectrum even at room temperature, which can be effectively bleached. In addition to the strength of the optical nonlinearity, a fast nonlinear optical response of the material is an important requirement in the development of optical processing in order to allow fast switching and high repetition rates at low switching energies.
Generally, in different optical switching devices, the on-off rate, or switching rate, thereof is determined dependent on response characteristics of the above-mentioned optical nonlinearity, and the switching time thereof also is determined dependent on the life time of the photoexcited electrons therein.
FIG. 1 is an absorption spectrum showing the relationship between energy of a beam and absorption factor when an input beam is introduced in a typical type of MQWS. In FIG. 1, reference symbol (a) indicates an exciton peak and reference symbol (b) indicates a starting point of the exciton peak. FIG. 2 is a cross-sectional view showing the construction of a conventional single MQWS where the absorption spectrum is presented.
With reference to FIG. 2, the single MQWS 1 includes a plurality of Al.sub.x Ga.sub.1-x As/GaAs layers each of which is composed of an Al.sub.x Ga.sub.1-x As epitaxial film and a GaAs epitaxial film. The plurality of Al.sub.x Ga.sub.1-x As/GaAs layers are formed on a semiconductor substrate (not shown) in such a manner that a wavelength of each beam to be used therein may be coincident with one corresponding to an energy level presented at the exciton peak, as shown in FIG. 1. For each beam to be used in the single MQWS 1, there is a control beam 3 and an input beam 4 having the same wavelength. The input beam 4 is switched by the control beam 3 in the single MQWS and transmitted through the single MQWS to provide a transmitted beam 5, as shown in FIG. 2.
In the single MQWS 1, each pulse width of the above-mentioned beams is set within one picosecond, and electrons in a ground state are photoexcited in the respective Al.sub.x Ga.sub.1-x As/GaAs layers. Then, a high energy state is created in the MQWS. As a result, the input beam 4, the property of which is changed by the control beam 3, is passed through the single MQWS, and thereby there arises a filling of the state of an energy level in the transmitted beam 5. By filling the state, a bleaching occurs in the single MQWS, and then the quantity of light is increased to a much larger extent in the single MQWS.
However, since the bleaching continues to arise only in the range of several nanoseconds up to several tens of nanoseconds by way of recombination of the photoexcited electrons, holes and lattices, the single MQWS has a slow response characteristic in the nonlinear optical effects.
FIGS. 3A and 3B are waveform diagrams showing photoreaction related to the optical pulses applied to the single MQWS when the exciton peak occurs. In detail, FIG. 3A shows the relationship between time and intensity of the input beam 4 or the control beam 3 shown in FIG. 2, and FIG. 3B shows the relationship between time and transmission of the transmitted beam 5 whose property is varied by the control beam 3, and which is transmitted by the control beam 3 in the single multiple quantum well.
As seen in FIG. 3B, by the bleaching in the MQWS, a curve of the rising time (or, ON time of switching) in transmission of the transmitted beam 5 is rapidly changed upward within several picoseconds, but a curve of the falling time )or, OFF time of switching) is slowly changed downward within several nanoseconds.
As has been described, in the single MQWS, the ratio of on-off times of switching is reduced substantially because of the slowly falling time. Therefore the switching speed of an optical switching device is limited in the range of several nanoseconds up to several tens of nanoseconds.
In addition, the above-described MQWS has a limited use in the field of an optical communication system where a switching time of within several tens of picoseconds are required and can be used only in an extremely low temperature. This causes many problems such as the limited use in the ultrafast optical communication system and the non-operation of the single MQWS under room temperature.